The present invention relates to an image processing apparatus and method and, more particularly, to an image processing apparatus and method used to perform image formation by using, e.g., electrophotography.
In conventional image processing apparatuses for forming color images by using electrophotography, an electrostatic latent image is formed on a photosensitive drum by irradiating light onto the drum in accordance with image signals of several different colors. This electrostatic latent image is developed by depositing toners of these colors on the image and transferred to a recording medium, thereby obtaining a full-color image.
In the above conventional development process, one of two-component development using a mixture of toner and a carrier and nonmagnetic mono-component development using only nonmagnetic toner is applied.
To perform the two-component development, the mixing ratio (T/C ratio) of the toner to the carrier must be held constant. If the balance of this mixing ratio is lost, the color reproducibility is degraded in the output image to lead to loss of image information. To prevent this, a controller for keeping the mixing ratio constant is necessary. Consequently, the construction of the developing unit is enlarged and complicated and becomes expensive.
In performing the nonmagnetic mono-component development, on the other hand, a device for controlling the mixing ratio such as the one used in the two-component development is unnecessary, since no carrier is used in the development. Accordingly, the construction of the developing unit is simplified, and, e.g., a magnetic field generating means for holding a carrier becomes unnecessary. This makes a small (lightweight) developing unit possible and maintenance of the developing unit easier.
Generally, however, the nonmagnetic mono-component development has the following drawbacks because only a DC voltage is applied between a photosensitive drum and a developing roller.
That is, in the nonmagnetic mono-component development it is necessary to decrease the charge amount of toner to about 1/5 to 1/20 the toner charge amount in the two-component development, in order to well deposit the toner on the photosensitive drum. Consequently, the adhesive force between the developing roller and the toner is weakened, with the result that the toner is scattered to contaminate the interior of the machine when the developing roller rotates.
Also, since the electric field between the photosensitive drum and the developing roller must be increased, it is necessary to raise the withstand voltage of the photosensitive drum. Accordingly, the photosensitive drum becomes expensive. The electric field can be increased by bringing the photosensitive drum and the developing roller close to each other. If this is the case, however, it is necessary to improve the mounting accuracy in order to keep the positional parallelism between the photosensitive drum and the developing roller. Consequently, a precise manufacturing process is necessary, and so an increase in the cost is unavoidable.
To solve these problems of the nonmagnetic mono-component development, various techniques have been proposed. Among other techniques the one disclosed in, e.g., Japanese Patent Laid-Open No. 63-231378, has solved the above problems. In this image processing apparatus, toner is held on a developing roller, and development is performed by superposing a DC component on an alternate electric field between the developing roller and a photosensitive drum.
When color images are formed by using the image processing apparatus described in Japanese Patent Laid-Open No. 63-231378, however, another problem as described below arises. Therefore, this apparatus has not been put into practical use yet.
When one type (one color) of toner is developed in the above conventional image processing apparatus, the parallelism of an electric field on a photosensitive drum is disturbed on the edges of an image. This is illustrated in FIG. 8. In FIG. 8, reference numeral 71 denotes a developing roller; 72, the surface of a photosensitive drum; 76, an electrostatic latent image on the photosensitive drum 72; 73, toner particles deposited on the developing roller; and 74, toner particles forming a visual image of the electrostatic latent image 76. In the above conventional image processing apparatus, the lines of electric force from the photosensitive drum to the developing roller are bent toward the photosensitive drum on the edges of the electrostatic latent image 76. Consequently, no toner particle adheres to a position 75 indicated by the broken line in the electrostatic latent image 76.
Since this phenomenon occurs on the edges of an image, inconvenience such as shown in FIG. 9B takes place in the formed image. FIG. 9B shows the result of the formation of an image, illustrated in FIG. 9A, in which rectangular patterns of colors Y, M, C, and K, each 10 mm in width and 40 mm in length, are arranged adjacent to each other in the direction of image formation (the rotating direction of a photosensitive drum: indicated by the arrows in FIGS. 9A and 9B). FIG. 9B shows that in the image formed by this conventional image processing apparatus, the leading edge, the trailing edge, and the central portion of the image, with respect to the image formation direction, have different image widths.
For example, when the image shown in FIG. 9A in which patterns of different colors are arranged adjacent to each other is formed of 600.times.600 lines/inch, the resulting image is as shown in FIG. 9B. That is, an image width of 10 mm is kept in the leading and the trailing edges of the image, but the image width is narrowed by approximately 60 to 120 nm in portions 100 .mu.m inside (image formation side) the leading and the trailing edges.
Also, when image information in which rectangular patterns of different colors are arranged adjacent to each other such as shown in FIG. 10A is output, an image in which the boundaries between these colors are blank is formed as illustrated in FIG. 10B.
A narrowing of the central portion of the formed image occurs each time the toner is developed, and so this phenomenon does not occur when monochromatic images are formed. In the formation of full-color images, however, if an image in which the boundaries between Y, M, C, and K are adjacent to each other is formed, no image is formed in these boundaries. As a consequence, the background color (e.g., white) of a recording medium is exposed (to be referred to as a white gap hereinafter). This results in low image quality.
This inconvenience is not limited to the colors of Y, M, C, and K. That is, when an image in which patterns of colors red (R), green (G), and blue (B) are arranged adjacent to each other is formed, white gaps are similarly formed in the boundaries between these colors R, G, and B.